This invention relates to a process for producing ceramic fibers and structures made with materials produced thereby.
Ceramic fibers have a variety of industrial uses including bioceramics such as bone implants, composite materials useful to enhance the high temperature properties of metals such as aluminum, catalysts, battery separators such as electrolyte supports, high temperature gas filters where porous structures are required to withstand high temperatures and in molten carbonate fuel cells.
Ceramic compositions having chemical stability and electronic conductivity at temperatures in the range of from about 500.degree. C. to about 700.degree. C. are important in the molten carbonate fuel cell. Molten carbonate fuel cells include an anode such as porous nickel, a cathode such as the porous materials disclosed in U.S. Pat. No. 4,574,567 issued Jan. 14, 1986 to Kucera and Smith, the disclosure of which is incorporated herein by reference, and an electrolyte such as combinations of lithium carbonate, potassium carbonate with lithium aluminum oxide particles.
Particularly useful in molten carbonate fuel cells are porous ceramic structures wherein the porous ceramic has a dual porosity. That is the ceramic has both large or macropores and small or micropores. The micropores are used to store electrolyte whereas the macropores are used to permit flow of gases through the cell. In such a ceramic for an electrode, it is desired that the micropores be present in a range of from submicron in size to no greater than about 5 microns. The macropores are preferably in the size range of from about 10 microns to about 150 microns. The macro porosity has been obtained in the prior art with the use of pore formers as shown in the patent in Swarr et al. issued Mar. 24, 1987, U.S. Pat. No. 4,652,411. In the Swarr patent, spherical agglomerates were produced and spray dryed in air at elevated temperatures. Because spherical agglomerates can pack more closely than fibrous agglomerates, the Swarr et al. method required the use of pore formers in order to provide the required macro porosity for use in a molten carbonate fuel cell. Pore formers are disadvantageous because they may leave residue and they do not always result in interconnected pores adequate for conducting gases through the structure.
Heretofore, ceramic fibrous materials have not been made without inclusion of a substantial majority of particulates. For instance, Paul A. Lessing reported in the paper entitled "High Temperature Fuel Cell Research And Development" published May, 1980, by The Montana Energy and MHD Research and Development Institute, Inc., a fibrous ceramic material. However, upon later testing by Professor R.E. Tressler, his photomicrographs show that the material was only 10% or less fibrous and the balance being bonded particulate material of random agglomerates. The present invention reverses the ratios, resulting in material which is more than 95% fibrous, that is substantially entirely fibrous.
Accordingly, it is a principal object of the present invention to provide a method of making ceramic fibers and the various products produced thereby.
Another object of the invention is to develop ceramic compositions useful in forming electrodes for molten carbonate fuel cells and particularly cathodes requiring dual porosity.